Bestseller Winspear’s sixth Maisie Dobbs novel (after 2008’s An Incomplete Revenge ) raises the stakes considerably for her psychologically astute sleuth. On Christmas eve 1931, a man Maisie passes on a London street detonates a bomb, killing himself and slightly wounding Maisie. This traumatic event turns out to be linked to threatening letters the British prime minister starts to receive, the first of which mentions Maisie by name. Maisie joins a high-powered investigative team devoted to averting the cataclysmic disaster promised by the unknown author of the messages. By providing the letter writer’s perspective, Winspear removes some of the mystery. In addition, Maisie’s speculative guesses about the profile of the criminal, while accurate, have less logical grounding than traditional puzzle fans might prefer. Still, Winspear does her usual superb job of portraying London between the world wars. (Feb.)